text:
Why would Boeing's MCAS be preferable to a modified Feel and Centering Unit?
According to some sources, the purpose of MCAS on Boeing's 737 MAX variants is to increase the back-force needed to further raise the nose when flying manually at high angles of attack, in order to give the airplane acceptable handling characteristics when approaching a stall.
Both the MAX variants and their predecessors, however, already have an elevator Feel and Centering Unit (page 8), the purpose of which appears to be the generation of appropriate stick force feedback in all stages of flight. If so, then, at first sight, this would be the appropriate unit in which to implement the function of MCAS, raising the question of why MCAS would be the preferred solution.
A few possibilities have occurred to me, but they are just guesses:

MCAS functionality needs angle-of-attack input, which may not be available where the Feel and Centering Unit is located (in the tail), and it would be complicated to get that information to it.
This answer states the the Feel and Centering Unit is a mechanical computer; given that, it therefore might not be easily modified.
Modifying the Feel and Centering Unit would require re-certifying it in toto, not just its new feature.
In addition to modifying the handling characteristics, MCAS is also seen as contributing to stall prevention directly, by reducing the angle of attack.

NB: Recently, Dominic Gates wrote an informative article in the Seattle Times about the origins of MCAS.

On the 737 NG, at high angles of attack, the nose of the plane would naturally pitch down, helping to recover from a stall and to increase airspeed.
The larger engine nacelles on the 737 MAX are located forward of the center of gravity, which means that at high angles of attack they are pushing the nose up. MCAS helps to push the nose down in this situation, similar to how the 737 NG would behave.
The added nose down trim has the side effect of requiring more elevator input at high angles of attack, but that was not the primary purpose.
There are several reasons that the Feel and Centering Unit would have a difficult time providing similar functionality:

The airplane should pitch down even the absence of control input, so it could not be done by simply changing the way the elevators respond to pilot input.
MCAS takes input from angle of attack, altitude, flap position, and airspeed. The Feel and Centering Unit currently just senses airspeed, so it would somehow need to get the other inputs.
The Feel and Centering Unit is a mechanical computer, so adding new inputs and changing the behavior could be very complex to design and certify.

Another possibility would be to use the Mach Trim system to adjust the Feel and Centering Unit. The Mach Trim system uses the flight computer to adjust the elevator neutral position to provide stability at higher speeds. While the flight computer should have all of the information needed, stabilizer trim provides much more control authority.

Are you saying the the handling characteristics issue that the Leeham News article discusses is not the reason for MCAS? Is the pitch-down behavior of the NG prior to the stall?

He’s not saying that the article is wrong. The handling characteristics at high AoA is precisely what MCAS is for. The FCU already increased aft stick forces when the stall warning was active in the NG. The difference is that, at a certain AoA the lift created by the engine nacelles became a factor, making a stronger nose up moment than the NG created at the same AoA. One selling point of the MAX is that it required very little transitional training for pilots currently flying NGs. They used the MCAS to more closely match the performance of the NG.

@TomMcW I don't disagree with the points you are making, but my question is not "what is MCAS for?"

I suppose the reason for the instability of pushing nose at high angle of attack is the center of lift is in front of the center of mass. But why does shifting the engine forward shift the center of lift? Without considering the secondary effect, the thrust of the engine is directed parallel to the fuselage and thus the wing. So there should not be a discernible difference in the torque with respect to the center of mass of the whole plane.

@Hans at high AOA the engines are producing aerodynamic lift. Thus the further forward (or larger) they are, the further that moves the total center of lift forward.

Your claim puzzles me in two parts. First, the engine itself produces lift in addition to the thrust parallel to the fuselage and wing. Second the lift thus produced is dependent on the AOA. Could you please elucidate those points or provide some references, best if they come with mathematical derivation?

@Hans All engine nacelles produce lift as a consequence of their approximately cylindrical shape. When the airflow is parallel to the axis of a cylinder, no lift is produced, but with the cylinder axis at some angle of attack, some lift will be created. As is generally the case, this lift is some function of that angle, the airspeed and density. Because the engines are forward of the wing, this lift is destabilizing.  For low drag, the engines are mounted so as to have a small angle of attack when cruising, so this lift is only significant at lower speeds and higher AofA...

@Hans To some extent, this affects all airplanes with engines mounted under / forward of the wing, but in the case of the 737 MAX, this has become a problem because the engines have become much larger and more forward than the ones the wing and stabilizer were originally designed in concert with. https://leehamnews.com/2018/11/14/boeings-automatic-trim-for-the-737-max-was-not-disclosed-to-the-pilots/ NB: lengthening the fuselage also destabilizes because a cylinder's center of pressure is forward of its geometric center. https://leehamnews.com/2018/11/30/bjorns-corner-pitch-stability-part-2/

A change in the pitch feel system wouldn't solve the problem.  It's the MAX's natural behaviour separate from the flight control system (that is, behaviour when you aren't touching the controls). As Fooot says, the MAX's engines have the effect of moving the overall center of lift forward somewhat, which is more or less the same thing as moving the center of gravity aft.
The airplane, in certain regimes (flaps up), ends up being neutrally or almost neutrally stable in pitch, especially at higher power settings where the thrust is contributing to the nose-up moment - bad enough that the airplane's pitch attitude would drift up when it should be rock solid, and worse, the natural pitch down you should get with a speed decrease wasn't there or was very weak.  It could be countered by the pilot, but the hand flying workload goes way up when you have to constantly intervene with an airplane that has a bit of a mind of its own.  Flying just about any airplane with an excessively aft CG is like that.
The proper fix would be to move the operating center of gravity range forward to cancel out the influence of the more forward engines and make the horizontal tail larger to compensate so that the tail power required to rotate at takeoff (which usually the tail's hardest job) is still there.  They didn't want to go that route and decided to use software to run the stab in the background to "mask" the stability problem from the pilot so that they could keep the C of G range where it was.  It's basically an artificial stability system with a narrow operating requirement added on as a band-aid to avoid a far more expensive modification.
It's not the first time it's been done.  I recall something similar was done on another type, namely the MD-11, that allowed the airplane to be operated with a farther aft CG than normal, to reduce tail down force in cruise, reducing trim drag.  I dimly remember an incident from long ago where the system was disconnected while in cruise and the pilot took over hand flying his neutrally stable airliner cruising at mach point whatever, and a Pilot Induced Oscillation got started that rattled people in the back around pretty good, like shaking a tube of Pringles potato chips.

This answer does not mention the feel/centering unit, but implicitly you seem to be saying that it would not be a candidate solution because, contrary to the Leehams News article, satisfying the handling characteristics requirement was not the problem that MCAS was developed to solve. If so, could you modify your answer to make this point explicitly, preferably with references to an authoritative source on what made some sort of mitigation necessary?

@sdenham, the problem with the centering unit is that it is designed to pull the yoke towards the center, but to augment stability you need to move the center around, which is what trim is for.

@JanHudec I am not suggesting that the Feel and Centering Unit, as currently implemented, solves the problem - if it did, there would be no need for any sort of modification to compensate for the effect of the new engines.

@sdenham, and I am not talking about current implementation, but overall purpose. The problem that MCAS was developed to solve is poor handling characteristics, but modifying handling characteristics is a problem completely unrelated to the purpose of the feel and centering unit.

@JanHudec According to https://www.737ng.co.uk/B_NG-Flight_Controls.pdf "The elevator feel computer provides simulated aerodynamic forces... Feel is transmitted to the control columns by the elevator feel and centering unit." This (and the name) suggests that it does more than centering, and modifying the control forces is what the Leeham News article says is the purpose of MCAS.

@sdenham: “modifying control forces” is too broad a term. Feel and centering unit provides “simulated aerodynamic forces”, but the aerodynamic forces always push the elevator towards the center (relative to the movable stabilizer), just increasing with dynamic pressure, so it still is “just centering”, with variable force. Changing the position of the center is why the stabilizer is movable, so adding logic that moves it is the more obvious change. There also were some other cases of auto-trim already since the previous model.

Feel and centering units are just boxes with bungee springs inside, and a mechanical system that has the springs work against a lever with a variable fulcrum. The spring pack will be driven by a lever linked into the control circuit.  When the pilot moves the controls he is compressing the bungee springs. The variable fulcrum is moved by a computer that adjusts it to change the force felt by the pilot, which simulates the increase and decrease in control forces with speed and other factors.The surfaces are driven by irreversible hydraulics so there is zero aerodynamic feedback to the controls.

@JanHudec According to Bjorn Ferhm, the MAX does not become statically unstable, and the specific issue is that "the aircraft OEM is asked to propose fixes to the pitch characteristics of the aircraft so it can be judged acceptable for use by a minimum standard (re. training and proficiency) pilot", and it was judged unacceptable (prior to MCAS) because, when approaching the stall, " the pilot would feel as if the aircraft suddenly pitches up faster for the same stick input." You may disagree with Bjorn, but if it is so, the issue is precisely the size of the force towards the center position.

@JohnK Thank you for your detailed comments. Is it unreasonable to say that, at least in principle, the variable fulcrum could be adjusted to correct the problematic 'lightening' of the controls at high angles of attack, by multiplying the restoring force by some factor > 1.0?  I am wondering if the computer that adjusts the fulcrum is a simple mechanical  (or hydro- or electro-mechanical) analog device, which I am guessing would be hard to add AofA input  and additional rules to, and might have to be completely re-certified for all flight regimes?

@JohnK With zero aerodynamic feedback (and thus with the stick force being entirely set by the feel/centering unit), the unit's job seems to be 1) have the stick free/neutral position correspond to the elevator being in line with the stabilizer 2) stick force is always towards neutral 3) at a given airspeed, a given change in force results in a fixed change in pitch 4) increase force with airspeed. Given theres no feedback, the reduced stability would appear as a large change in pitch for a small displacement change, which would feel 'light' because the force is proportional to displacement.

The root issue is the behavioural problem is not something that can be addressed by changes to the force regime felt by the pilot.  The issue is  "external" natural behaviour you might say, that required "external" intervention by a system that operates in the background and forces the airplane to behave, in a way that is transparent to the pilot.  Using the stab screw jack is the easiest way to do it.  An alternative could be a device that works sort of like a yaw damper, moving the elevator downstream of the feel system so the pilot is not aware of it, but that would be way more expensive.

@sdenham, I did not say it is statically unstable, but what you describe sounds exactly like there is a region of insufficient static stability just before stall. As the angle of attack increases, the centre of pressure—on every aircraft—moves slightly forward, creating pitch up moment and reducing static stability. If that's indeed the effect that became too strong here, it requires creating a pitch-down moment, i.e. trimming nose down. Without that the pilots can simply trim for the dangerously slow speed and multiplying the force won't help, because it will be zero in the first place…

@JanHudec You will see stated earlier in the same article, reduced stability at high angles of attack is indeed the cause of the unacceptable handling characteristics. While your issue of pilots being able to trim out the restoring force seems valid, it would also seem something you could say about any airplane approaching a stall. It is also true that pilots can use the trim switches to countermand ACAS, so ACAS does not eliminate the risk. (continued)

@JanHudec  When you say "it requires creating a pitch-down moment", you seem to be saying that ACAS is intended to be a sort of pre-stall stick pusher? If you think that is the purpose of ACAS (something that the feel & centering unit could not do), perhaps you could write it up as an answer.

The problem as I understand it was that the airplane when at low speed flaps up could start to pitch up and slow down without any pilot input.  It would drift below its trim speed in other words.  MCAS applies a subtle correction by running the stab nose down about 9 sec, then stopping and waiting to see what happens.  If airplane is still pitching up it repeats the process.  I don't believe the '37 even has a pusher.

@JohnK What you are saying here is that it becomes statically unstable at high angles of attack. I have seen some sources (or opinions) saying that, but others (including Bjorn Fehrm's article) saying no, it is just reduced stability. This is just one issue surrounding MCAS where there are more opinions in circulation than facts, and maybe I should first ask about them... If 737s do not have a stick pusher, that might be an argument for MCAS, as MCAS has pusher-like behavior.

@JohnK Let me rethink this somewhat... airplanes will continue to slow down (and consequently have an increasing angle of attack) if they are initially trimmed, on the back of the drag curve, are disturbed in such a way as to decrease the speed momentarily, and no corrective action is taken. This is not considered to be unacceptable behavior, at least in general aviation (I'm not sure where this observation is going...)

@JohnK I found another source (though not primary) saying the issue is reduced force, not full instability: "As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. This abnormal nose-up pitching is not allowable under 14CFR §25.203(a) "Stall characteristics".' Also '[MCAS] is not for stall prevention (although indirectly it helps)'  http://www.b737.org.uk/mcas.htm ... I have also seen FAR §25.173 (stability).

You support @fooot's opinion and claim that "the MAX's engines have the effect of moving the overall center of lift forward". I doubt that and have posed it to him below his answer: "Your claim puzzles me in two parts. First, the engine itself produces lift in addition to the thrust parallel to the fuselage and wing. Second the lift thus produced is dependent on the AOA. Could you please elucidate those points or provide some references, best if they come with mathematical derivation?". However, he is unable to answer it. Do you have an answer to my question?

@JohnK -- the McD aircraft you're after for the pitch stability issues is the MD-11 aka the "Pitch B#$!@ of Long Beach", BTW

Boeing wanted the MCAS effect to be transparent to the pilot, as a proof nothing about it was mentioned in the FCOM.
Acting on the feel and centering mechanism would require a sudden effect on the column which would have been noticed and declared as a fault by the pilots since to get the same effect as does the trim, considering the elevators area compared to the THS area, you would need a sudden tremendous visible displacement of the column centering while a single trim shot of 2.5° ( .6° in the original design) is less disturbing, mainly because short trim movements are possible for other reasons even though in manual flight, such as Mach trim.
Who will bother much about a single trim shot, was it not a faulty AOA producing repetitive trim shots? As a matter of fact it remained transparent till the faulty AOA producing the crashes

Thank you for replying, but could you expand on why it would inevitably result in a sudden effect? Would it be impossible for the unit to progressively increase the force, relative to that produced by the NG's unit, as the airspeed decays towards the stall? (I say speed rather than AofA, because, AFAIK, the unit does not get that data, but it does have a pair of dedicated pitots.)

@sdenham, Increasing the force makes a difference with the NG thus the need for training but Boeing didn’t want extra simulator training, thus the need of MCAS and the most transparent possible.  It is sudden because it occurs unexpectedly at flap retraction once speed is above 230 knots. Please refer to the following website.    https://www.reuters.com/article/us-ethiopia-airplane-regulator-insight/regulators-knew-before-crashes-that-737-max-trim-control-was-confusing-in-some-conditions-document-idUSKCN1RA0DP

How would using the feel and centering unit be seen as a fault by pilots, but using trim wouldn't be?

The root of the problem with the MCAS is that the Angle of Attack sensor is near the nose instead of on the leading edge of the wing, where it belongs. It's supposed to be measuring the Angle of Attack OF THE WING. Instead, it measures the Angle of Attack OF THE NOSE. These measurements are not the same while the aircraft changes pitch.
With the long fuselage, bringing the nose down from climb to cruise will falsely indicate a nose-up attitude because the fuselage is rotating around the wing. This is why both crashes occurred from problems that arose when changing from climb to cruise. Whether the MCAS malfunctions or not depends completely on the rate at which the pilot changes pitch. Push forward too far on the stick, and you are doomed, when the MCAS takes over pitch control and will not give it back.
The false Nose Up reading moves the elevator trim to further lower the nose, again triggering a false nose-up reading. It's a feedback loop that will not stop until MCAS hits its limit, which gets reset every time the pilot follows instructions and presses the reset button. Press it more than three times, and trim is set to the mechanical limits of the elevator trim. In the second crash, the pilot hit the reset button more than 20 times.

But near the wing you have side effects from the flow around the wing. There are plenty of alpha vanes on fuselages, e.g. on all airbus aircraft that I know of. It would also be possible to correct that error mathematically with the pitch rate and airspeed. If you move the angle of attack sensor to the wing you'd have to calibrate it for all different flap settings, with and without failures of either flaps or slats. Much easier to just calibrate it on the fuselage where at least the geometry doesn't change.


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Does a runway have to be closed for not having markings due to resealing work?
Is it required to close a runway if the paint has been removed for resealing the asphalt?

This is an interesting question! If you are able to, it might be helpful if you could provide additional information for your question, such as whether the airport is public or private, or what country or jurisdiction you are wondering about.

Not Necessarily
While some regulations or guidance material may call for some or most runways to be closed during maintenance or construction work, such as marking removal for asphalt reseal, this is certainly not always the case.
If markings are non-standard, faded, or removed, for any reason, the condition should be documented in a Notice to Airman (NOTAM) so that pilots and other interested parties will be aware of the condition and prepared for the non-standard situation.
An example of such a NOTAM includes this one from Randolph AFB (KRND), where some markings on runway 14L/32R are missing dues to removal and repaint during maintenance or construction work.

RWY 14L/32R SURFACE MARKINGS NON STD ; SOME MARKINGS MISSING DUE  TO
  REMOVAL AND REPAINT.

Note that runway 14L/32R is not closed during this work. However, the parallel runway, 14R/32L, is closed:

RWY 14R/32L CLOSED

IT relies on being a Part 139 airport, mainly. the Marking Standard, AC 150/5340-1L states the guidelines and standards for marking are the only method of compliance with airport certified under Title 14 CFR Part 139. Part 139 is the certification and clasification of passenger airports. Markings are also required for any airport receiving Federal grant money through the AIP Program. Basically, if you are not a passenger airport with scheduled or non-scheduled service of aircraft with 30 or more passengers, or if your sealing project is not funded through a Federal AIP grant, you may leave temporary markings off as long as NOTAMS are issued to this effect to the pilots - that the runway is unmarked and they are on their own discretion whether to use or not. At least, this is my understanding. See the AC. I should also point out the "Operational Safety on Airports During Construction has the same compliance requirement - AC 150/5370-2L - applying to Part 139 and AIP grant program airports. This is why you may see non- standard markings or no markings (grass field) on private airports.
But there is a secondary consideration - State Funding Grants. You may need to check with the State Aviation Responsibility if they are paying in part for the work as if you need to meet the FAA requirement. Some states may be lenient due to cost. State grants are applied on a matching basis, and it may cost a local GA only 1/10 the cost when receiving a State grant for the work. Since even a simple seal and repaint project can cost $100K or much more, the decision to close a runway may rest with the State agency. You see, most GA's no matter how small are eligible at times for FAA AIP or State matching grants, and this drives what they do, and why most non-commercial GA's still meet the FAA AC's for marking and construction safety.
Any corrections gladly welcomed.

Welcome to Aviation.SE! Not a bad first answer, if you haven't already, I would invite you to take the tour and read the [help] to see what makes a good/bad answer.

Yes, of course you have to close it since it falls under the runway closure reason of "Obliterated or faded markings on active operational areas." From this document. The "Obliterated markings" is what would cover the removal of the markings for any reason.
And with even simple common sense, why would a runway under construction be even open?

I'm not entirely sure the "you have to close it" bit is correct, at least in FAA-land (What regulation mandates closure? That AC says to "consider" potential hazards). It's unsual to have a runway open with no markings, and if it were open a NOTAM would be expected indicating it was unmarked and noting any applicable restrictions (an unmarked runway couldn't be used for instrument procedures for example), but I can't find a regulation that requires closure...

Of note, the old quick reference for airfield standards (from 2011) used to include procedures for opening an unmarked runway. The current edition (2015) no longer includes that language though.

Advisory Circulars such as that which you reference are advisory, not regulatory. 2) While some or all public use airports may need to be closed due to regulatory requirements or might be closed in this circumstance due to prudent choice, I can imagine many situations—including at public use airports—where leaving a runway open despite having not markings would be beneficial to the public good. Construction projects can occur in many phases, and marking removal might well be followed by a period of inactivity in which the runway could be used.

the AC you cite does not give "obliterated markings" as a runway closure reason, but as an item to look for in safety inspections. In reading that AC I have yet to see any justification for this answer. 4) The AC you cite does specifically, and at length, specify when and how a runway under construction may remain open or partially open.

This answer seems to be specifically about FAA regulations, even though the original question does not specify which country is being asked about.

source: Stack Exchange

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text:
DME replacement
I need help with this question:
I have an ancient DME in my airplane, a "KN 60C" and it turns on and the dial bounces and sits at 25 miles, so my guess it's time to swap it with a digital one.   Problem is money is tight and I'm hoping somebody can recommend a model of DME that would be the closest thing to a swap replacement.   I see allot of KN 64 and Narco brands on EBay and I need to know which one would work with my current antennas and such.  Of course the avionics shop would install it, but I can't seem to get their interest in doing this.  I just wan to walk in with a specific install request.   Someday I'll get a 430 GPS, but for now I'm happy with my dual glide slopes and ADF, I just need a working DME, please help

Hello and welcome to Aviation.SE! Hardware requirements aren't on topic for this site. These questions are normally either opinion based or should only be answered by a qualified person. You can see what is on topic in the [help].

Just buy a few spare KN60Cs and slide them in when the one you're using breaks.  A quick search shows a "working when removed" 60C costs about $200.  A kn64 will cost $1400 plus the cost to install the new tray and harness, so I'd guess at least $2000 to convert over to the more modern, but still obsolete, kn64.  It's too bad this is "off topic."

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


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Do fighter jets have a traditional inverted fuel system?
Common solutions for aerobatic piston singles is to have either header tanks (for wing tanks, as I understand it) or flop tubes (for fuselage tanks).
Do fuel systems in a fighter jet work on the same principles? Or are they somehow smarter to allow more erratic maneuvers?

I don't know for certain as I'm not military, but I imagine that they pressurize the fuel tanks using bleed air, much like jet airliners do. It's also likely that they make use of boost and ejector pumps to provide positive pressure directly to more centrally-located tanks which the engines then feed from.

(from FAA AMT Handbook [pdf])
The F-15 has both wing & fuselage tanks; according to a site of dubious accuracy (F-15E.info):

The left and right engine feed tanks contain baffles in order to provide a limited amount of fuel to the boost pumps during inverted flight or during negative G maneuvers

Left wing tank
Auxiliary tank
Left engine feed tank
Right wing tank
Right engine feed tank
Tank 1 (main tank)

Baffles are dividers in the tank itself that limit the speed at which fuel can flow from one end to another, usually to prevent fuel sloshing and screwing up balance.

Pressurizing the tanks to push the fuel in makes sense. I see how baffles would prevent sloshing, but how can they prevent the boost bumps from sucking in air during a negative G maneuver or inverted flight (being technically the same as -1 G)?

@roe I think it's basically that they prevent the fuel from immediately sloshing to the bottom of the tank (sloshing can include top to bottom, right?).

From what I've heard by asking around, the baffles compartmentalize the fuel tank, and restricts the flow between each compartment. Basically, it works like when turning a water bottle upside down, unless there's a hole in the bottom (now top) of the bottle, air is going to have to flow in as water flows out. The compartments are basically a bunch of chained bottles, air would have to flow into the first one, before being able to reach the second and so on. There is a whole on the other side, but it's one way (the pump), so fuel flows a lot better in that direction.

Some military jets do indeed have simple inverted pickups, but most modern ones have more advanced fuel systems


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Could a patient be too large and too heavy for an ambulance helicopter?
On the German news website BILD, there is a short video clip about a patient near Lauterbach being too large and with 150kg too heavy for the helicopter. Another source.
Images on the site show it has D-HUTH written on top, which is the registration for a model EC-135 P2i.
How much of the story is true and how much is overexaggeration?
Since I got asked for references:
Normal article, no mention of the size/weight problem, the guy in the video only mentions it when they tried to get him out of the car:
https://m.osthessen-news.de/beitrag.php?id=11674876

I can't answer this question directly because I don't fly that helicopter, but I can say that in generic air ambulance work, our hardest task is getting the patient + stretcher through the door.

The article says that the man did not fit into the helicopter due to his weight and size. I think the bigger problem was not lifting capacity, but getting the patient on the stretcher into the helicopter, both due to their size and due to the weight. Remember that someone has to lift the stretcher into the helicopter.

It's worth remembering that it is extremely common that news articles are simply: often hopelessly incorrect.  Reporters completely muddle words or concepts. This is essentially the normal state of affairs.

@Fattie BILD is a paper which is a bit special. There is an old joke: 50 people die in an accident - BILD talks first to the dead.

People can be too heavy to leave their house without cutting the wall open and using a crane. Darlene Cates "known for her role in the 1993 film What's Eating Gilbert Grape, in which she played the title character's housebound mother."

150 kg being too heavy to lift for an ambulance EC-135 is not very believable.

Screenshot above is from this site, stating a max payload/fuel weight of 1,375 kg with one 80 kg pilot on board. With a maximum fuel load of 560.4 kg, this leaves 814.6 kg payload capacity.

The advanced rescue EMS kit is depicted on page 56, including 2 seats, a stretcher, life support & oxygen systems etc, and is stated to weigh 142.1 kg. With 2 medivac personnel plus a co-pilot of 80 kg each and maximum fuel on board, patient weight capacity would be 432.5 kg - a lot more than the 150 kg that supposedly Bild claimed was too heavy for the helicopter. (Tried to verify on Bild.de but did not want to agree to advertising cookies).
The EC-135 is a twin engine helicopter that can continue flying and landing upon a single engine failure - even taking off with One Engine Inoperative is possible with a reduced gross weight of 2,870 kg @ ISA + 20°C. So even with OEI, a patient of 150kg would be no issue.

Another possible problem could have been the physical size of the patient. The payload section of the cabin interior measures 2.5m long and 1.5m wide. It is possible to place 3 seats side-by-side - at 50 cm each that would leave about 1 metre width for the patient. Loading a stretcher with 150 kg on board could be an issue with handling - but there are systems designed for handling patients over 300 kg (bariatric patients), as mentioned on this site:

“On the rotor wing operation, Ornge has invested in ensuring we have the right aircraft with the right equipment to maximize support for bariatric patients,” said Pyke. “Our AW139 helicopters are well equipped to handle bariatric patient operations. We have partnered with HeliMods to install a PAL system based on Stryker power equipment. This provides a no-lift solution to loading and securing of patients up to 700lb (318kg), depending on clinical equipment requirements.”

----Update----
The linked RTL article states in the headline that the patient was too heavy, in the article itself mentions (translated from German):

According to the fire brigade, however, the helicopter flies back without the man, as the 30-year-old does not fit into the helicopter due to his weight and size.

So there was a size and weight issue - indeed perhaps a stretcher issue as @GremlinWranger suggests in a comment, however not a payload issue.

Is there in data on the limits of the stretcher or securing system? Issue may have been safely moving and loading the patient. A quick check suggest aviation stretchers are rated to around 180kg, but there may be additional issues with door height or size in play here.

+1 for citing the no-patient weight and remaining payload.

How much fuel would the helicopter have burnt on the way to the accident site? Obviously, since it flew to the accident site, it was capable of lifting off and flying with that extra weight as well.

@JörgWMittag The RTL article mentions that the heli arrived from Siegen, 150 km distant. The Eurocopter link specifies cruising speed of about 250 km/h @ 234 kg/h, so about 150 kg

And what about altitude?  Around here most of the choppers can't even reach the top of the local mountains.

@LorenPechtel According to the wiki, Lauterbach in Hesse is @ 500m altitude. Twin-engine hover ceiling for the EC-135 is stated as 3,460m on a hot day.

You can see the rear door's size at around 8 minutes in this video: https://www.youtube.com/watch?v=DE77bAup2xk (EC-135 ambulance variant in Hungary)
Add the stretcher's height to the patient, and you can see the space is very limited.

Former helo pilot here.... I agree with the above regarding the overall weight of the aircraft probably not being an issue.  Could have been the size of the patient, as mentioned.
But I have two more ideas:

Everything in a helicopter has to be tied down to the airframe in a way that makes a crash survivable.  It could be that the stretcher and its associated tie-downs are only rated up to a certain total weight of stretcher + human.

There may be a requirement that the flight medic be able to move the patient himself to deal with potential in-flight emergencies and the weight limit is actually due to human factors.

As a former Red Cross volunteer helper (never coming close to the helicopters, though), I remember that our ambulance stretchers were only certified up to something like 130 kg weight of patient. Maybe that played in.

Thanks for providing the links.
The EC135 is a fairly large helicopter. No Mi-26, but it's not an R22 either.
Any helicopter of this size is well capable of lifting a larger than average any human.
150kg isn't the kind of weight that could cause such an issue. The flight must have been planned for a 80-100kg person, so the extra 50kg is within the margins.
According to the article,

The maximum take-off weight of 2910 kilos would have been exceeded.

This explanation does not ring true.
The helicopter has taken off from somewhere, before landing at the accident site. It has used up some fuel during the flight. The amount of fuel used getting to the accident site would very likely be 150 kg or more.
Even if the helicopter took off at maximum load and the accident site was very close to its takeoff point, steps can be taken to mitigate it. One of the paramedics can step off the helicopter, for instance.
The MTOW isn't absolute, but a general limit for most cases. It is affected by weather and altitude.
Lauterbach is below 500 meters. The weather in Germany on the day of the event was around +10C. This means full engine power is available and a bit more if necessary (intermediate contingency). When addressing a medical emergency, it's acceptable to operate at the margins.
There might have been some other issue preventing the patient from being loaded into the helicopter. It wasn't the risk of exceeding its MTOW.

"The amount of fuel used getting to the accident site would very likely be 150 kg or more." – According to Koyovis's comment, the fuel burn for the flight to the accident site would have been almost exactly 150kg. So, if they couldn't take off with the patient on board, then they would also not have been able to take off with the extra 150kg of fuel to get to the accident site in the first place!

@JörgWMittag: That simplification does not account for potential altitude differences between the take-off site and the location of the patient. If the patient was located in a mountainous area, the heli could be much closer to its service ceiling and unable to take off with an excessive load.

As stated by other anwers, the situation was not about weight. With pilot, full fuel and two medical personnel, regardless of the equipment packages there would still be at least 200kg of payload left. The Helicopter was not fully fueled as it had just flown 150km to the accident site.
The reporter has either misunderstood information given, or was given a wrong information.
Most likely this is an issue with the physical size of the patient. A person weighing 150kg is about twice my weight. I'm average size, so if I imagine a severely overweight person with considerable amounts of extra bodily fat (fat tissue is less dense than muscle), the sheer size of the person will be problematic when loading onboard a helicopter. If the person is shorter than average, the problem is emphasized.

Well, for example that is an 155 kg man: https://i.dailymail.co.uk/1s/2021/05/19/15/43181769-9596281-image-m-35_1621434330518.jpg Would you think that this is an overweight/fat issue?

Well if we look at BMI, he would be overweight  While that guy is not fat, the problem with fitting him inside the chopper would be the same.

It just might be the stretcher - the ones I checked are only rated for 150kg so quite likely this might have been the main issue.

I don’t know about Germany but here in Austria I think they are using (or at least were using) the Stryker stretchers which are rated up to 227kg.

I admit my search was not the most complete, but that were the first results I found - and one of the possibly valid information for these kind of incidents.

It's worth mentioning that 150, 200 and 250 kg are semi-standard "max weight" placards on medical equipment. I professionally work with developing medical imaging techniques and 150 kg and 250 kg are by far the commonest weight limits for the patient tables on both CT and MRI scanners.

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text:
What does the term "trimming" most commonly mean in aviation?
Why does an airplane need trim, and what does it do during the flight? Does an autopilot adjust the trim automatically?

The purpose of trimming is to free the pilot from having to exert a constant pressure on the controls. This is often used to maintain straight and level flight, however trimming can also be used at any phase of flight - for example to maintain a constant rate of climb or descent.
The most basic form, as found on most light aircraft is elevator trim. Usually operated by a wheel, it moves the elevator up or down by a small amount in the same sense as the yoke (back to go up, forward to go down). This can be used to settle the aircraft into straight flight.
Another form of trim is rudder trim. Often found in larger light aircraft and twin-engined aircraft it can be used to adjust for a crosswind to keep the aircraft flying straight. On multi-engine aircraft it can also be used to trim out the differential thrust caused by one engine failing.

Most autopilots do trim. Also, trimming is not just for straight and level flight. It's used in every phase of flight, including climbs and descents. In fact, it's even more important to trim correctly during climbs and descents.

@PhilippeLeybaert - Very true. Answer updated

Interestingly, I've also heard this same term used in submarine operation, in relation to climb and dive.  One trims the ballast

Rudder (trim) is not used to adjust for crosswind (except on final approach, but that's another topic)

Trim is also needed to maintain constant attitude with changing performance.  E.g. as speed increases, without any control input, the nose will tend to pitch up and the aircraft will climb.  Concorde automatically moved it's fuel around to compensate for changing performance and shifting CofG.

I've never heard of trimming to adjust for a crosswind!  (Also, you forgot to answer the portion of his question about the autopilot.)

In the small airplanes I fly, we adjust the rudder trim for cruise only (and typically it is already set where it should be this is the whole point no need to touch it unless it's off). Every other phase of flight is just dealt with by rudder pedal pressure. The opposite is true with elevator trim, it is used for climb, cruise, descent, and essentially anytime the aircraft needs to be stabilized at a certain airspeed.

In short: trimming neutralizes the force required to keep control surfaces in a specific position.
Most (if not all) aircraft have some sort of elevator trim control. For example, when the pilot has to keep pulling back on the yoke/stick during a climb, trimming "nose-up" will neutralize that force. The elevator will then remain in the same position without any force required on the part of the pilot. Trimming is usually done by means of a Trim tab on the control surface and is controlled by a trim wheel.
Elevator trimming is used:

During climbs: to maintain a constant air speed and rate of climb
During descents: to maintain a constant air speed and glide path
During level flight: to maintain altitude and speed

Although elevator trim is the most common, trimming can also be done on rudder (quite common) and ailerons (only on larger aircraft).
Rudder trim is used to maintain coordinated flight without rudder input by the pilot. Many single engine planes with powerful engines require rudder trim to offset the "left-turning tendency" caused by P-factor and propellor wash hitting the rudder. Aircraft without adjustable rudder trim will usually have a fixed trim tab on the rudder.
Most autopilots will control the elevator trim wheel because the servos controlling the elevator could easily be overpowered by the required force.
It's important to note that in most cases (especially on small airplanes) trimming doesn't actually move the control surfaces. It changes the force required to deflect the control surfaces. Although there are many types of trimming devices, the common trim tab makes the airflow do all the work. The trim tab causes the airflow to push the control surface in a specific position. On larger aircraft with hydraulically operated control surfaces, trim tabs are less common (see also this answer to another question)

Trimming is changing the stability of an aircraft in such a way that when no control inputs are given, the aircraft maintains its attitude and speed. It allows the pilot to release the control input without the aircraft deviating from the intended path. In general the aircraft is trimmed for straight and level flight, but it can also be trimmed for descent or even for turns.
There are various ways in which an aircraft can be trimmed.
Trimming most commonly refers to elevator trim. This involves changing the angle of incidence of a trim tab on the elevator in such a way that when the control column is released the aircraft does not pitch up or down by itself. In large aircraft, the angle of whole horizontal stabilizer can be changed to trim the pitch tendency of the aircraft.
Aircraft that have a fuel tank in the tail can move the centre of gravity by transferring fuel between forward and aft fuel tanks. Use of CG trim not only affects the pitch tendency of the aircraft but also reduces the fuel burn since less aerodynamic forces are needed to balance the aircraft, thereby reducing the drag.
Other forms of trim are aileron trim and rudder trim. These are used to nullify  any asymmetric characteristics of the aircraft. These characteristics can be caused for example due effects of propeller wash, mechanical deformation of the aircraft (or poor manufacturing tolerances) or engine out situations.
On most larger aircraft the autopilot uses elevator trim or horizontal stabilizer trim to control the aircraft in pitch in addition to elevator control. For turning, ailerons and rudder are used, the asymmetric trim is not used by the autopilot.

Nicely put, but I wouldn't say that trimming "changes the stability" of the aircraft.  If it is out of trim, it is still stable.  It just wants to stabilize out somewhere other than where you want it to.  :)

@Lnafziger nowhere did I intend to imply that an untrimmed aircraft is not stable. I considered adding a sentence about ensuring that the poles are always in the left half of the complex plane, but I think that won't make it clearer for most readers. (you have a background in engineering don't you? :-)

Haha, yes I do, but the very first sentence of your answer says that "Trimming is changing the stability of an aircraft...".  Just thinking that there is probably a better way to say that.  :)

Autopilot theoretically could not care about the trim but if you disconnected it and plane was out of trim in could depart flight path rapidly.

"Theoretically" it doesn't care about trim, but the servos of the autopilot are usually not powerful enough to control the elevators if not properly trimmed.

Say you have trimmed an Aircraft than forces on aircraft i.e Lift == Weight & Thrust == Drag. So aircraft is flying (straight and level) or climbing or descending at constant speed or rate(of climb or descend).
Simple to remmember, Trimmed Aircraft means Lift = Weight and Thrust = Drag thats it.
There are lot of way to trim an aircraft: Trim Tabs on control surface, complete surface moves to trim corresponding control surface (ex: stabilizer moves in response to pitch trim command) etc.

Lift != Weight when climbing or descending.  :)

@Lnafziger Now you've opened a can of worms...

@Lnafziger: Yes, during change of phase of aircraft (climb, cruise, descent etc) these forces will not be equal but during constant rate of climb/descent these forces (Lift = weight or Drag = Thrust) are equal. I usually remember trim to be analogous to inertial i.e unless there happens to be some change [rate/direction/..] on aircraft, its always in trim state.
Also, the below article can explain it much better : http://www.pprune.org/archive/index.php/t-268982.html

Trimming isn't a matter of equalizing lift to weight (for instance), it's a matter of equalizing forces on a control surface, by creating very little lift on the trim surface. This small force combined with the distance between the rotation axis of the control surface and the aerodynamic  center of the trim surface is sufficient to cancel any torque from the control surface, hence any feedback on the control used by the pilot. The trim force could be created using hydraulics (no trim surface, no lift change) as well, the result would be the same.


text:
Is the aircraft used by NOAA specially modified?
I have just seen this short video showing "NOAA hurricane hunters" flying through current hurricane "Irma".
My guess is that normally an aircraft would not be able/allowed to fly in winds sustained wind speed of 160kts, gusting to 180+kts, with the associated turbulence.
Is this aircraft modified in any special way? For example, reinforced structure, or specially designed engines?

Aircraft fly through (relatively) stationary at speeds significantly higher than 160kts. Why would flying through air moving at 160kts be any different? They'd just have a lower ground speed for any given air speed.

An aircraft only cares about how it's moving with respect to the air it's in. If you had a completely smooth wind of 160kts, the aircraft moving in that wind wouldn't care at all how fast the air was moving relative to the ground -- the plane isn't interacting with the ground. The problem for flying into hurricanes is that the wind isn't remotely smooth.

If it had a steady headwind of 160kts, it could hover.  Landing would be hard, though - hitting the land at a sensible speed is easy enough, it's staying down in that wind!  That said, SuperSTOL style stunt planes with extremely low stall speeds have been known to do it.

"My guess is that normally an aircraft would not be able/allowed to fly in winds sustained wind speed of 160kts." Actually, that's not much different from the jet stream winds that airliners intentionally fly in routinely. Those are relatively smooth and in a more-or-less straight line, though, not spinning and turbulent with intense convection. I've seen tail wind hit 180+ mph on trans-Pacific flights that I've been on before. This is why LAX->HKG took 15 hours, but HKG->LAX took only 12, even though it traveled a longer distance.

yes, sorry everyone, I should have focused on the gusts (that is what I was actually thinking of).

There is some room for interpretation in what one means by "modified". A militarish aircraft can be described as "modified" from the beginning, when one looks at all aircraft (which are mostly civilian), in sense that vanilla Orion already is a "modified Electra". Yet, it's certainly "not modified" when considered among Orions. And again, when you look at military aircraft in general, transports are pretty weak compared to fighters.

Except for modifications related to instrumentation, not much. The WP-3D was developed from the Lockheed P-3 Orion, which was  a navalized L-188 Electra.

Orion is a sturdy platform. Used during the Cold War as an antisubmarine patrol aircraft, the plane is built to take punishment.

As the NOAA noted in a Reddit AMA

The WP-3D is built like a tough old pickup truck. It can handle a lot of abuse in the storm environment and come back and fly the next day. This is mainly because of the extremely talented maintenance team at NOAA’s Aircraft Operations Center (AOC)

Simply put, it is a normal aircraft with extensive instrumentation, albeit maintained well. The same applies for the WC-130J used by the 53rd WRS:

The 53rd WRS uses the WC-130J to penetrate tropical storms. These aircraft are not reinforced in any way in fact the only differences between a WC-130J and a C-130J is the addition of two external fuel tanks (giving them longer range), a radiometer pod on the left wing and the two addition crew pallets in the cargo bay...

and of course, the instruments. Actually, the NOAA has replied to your question in their FAQs:

Why aren't NOAA's Hurricane Hunter planes torn apart in storm?
Planes are generally not destroyed by strong winds while in flight. Airliners routinely fly in jet streams with winds exceeding 150 mph over the U.S. during the winter. It's the shear, or sudden change in horizontal or vertical winds, that can destroy an aircraft, or cause its loss of control. That's why NOAA's Hurricane Hunter aircraft don't fly through tornadoes. In a like manner, NOAA pilots and crew routinely (but never casually) fly in the high-wind environment of the hurricane and don't fear it tearing the plane apart. However, they are always monitoring for "hot spots" of severe weather and shear that they can often identify on radar and avoid if it's too severe.

The simple answer to your question is no - the aircraft is not structurally modified.  However, to add some context to @aeroalias's answer, here is a great visualization of windspeeds at various altitudes**.  At 1000hPa pressure altitude (approximately ground level) the scene looks like this (see Irma just over the Dominican) :

Now if we go up to 500hPa pressure altitude (~18,000ft) the situation looks very different.  The lower levels of the jet stream can be seen and the wind speed in the hurricane, other than being quite localized and subject to more shear, isn't really anything unusual in terms of magnitude.

It's not clear how high the NOAA plane was, but it looked to be at very least over 10k ft, I think.  At 500hPa, also, we can note that the lower pressure means that the instantaneous power density of the wind is halved at the same wind speed.
By 250hPa (~34,000ft) other high level winds dominate the sky and the hurricane is little more than a mild bump:

Now, clearly there are differences between the extreme wind velocities in the jet stream and those surrounding a hurricane.  Updraft, shear, and roll formations exist throughout a hurricane and these are markedly more dangerous than clear air turbulence at the jet stream boundaries.  A hurricane is quite large and regular, however - its wind patterns are not so extremely changeable and this means it is not entirely unnavigable.
Arguably, large thunderstorms, for example, can be more unpredictable and dangerous to an aircraft than a hurricane because of the randomness of their interior.  Not that you'd want to fly through a thunderstorm either, but at the right altitude and with sufficient care and caution it's not as mad a proposition as it may seem to embark on the NOAA's flight path through Irma.
Most aircraft can safely withstand much heavier turbulence than pilots ordinarily like to fly through.  It does carry risk and it does put more stress on the airframe - surely these scientific planes get more regular and critical inspections than commuter craft simply due to the higher stresses their flights would cause - but it is clearly within the performance envelope of the plane to make such flights.
Images all sourced from https://earth.nullschool.net/.  Definitely an excellent reference.

This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review

@Gerry I suggest you familiarize yourself with the guidelines for reviewing

This answers the question fantastically - it's an amazing explanation showing that the effects experienced "in a hurricane" are actually trivial compared to "normal high altitude flight".  What a great answer.

@Fattie I wouldn't say "trivial", to be clear - high altitude winds are very fast but generally smooth.  There is turbulence when crossing into and out of the jet stream, for example, but hurricane winds are significantly more changeable which does make them more dangerous.  It's a risky flight through a hurricane at mid-altitude, but it's very much a calculated risk rather than reckless and suicidal.

I always find hurricanes to look deceptively peaceful on such charts, like here (.gif of Irma).

Yes one of the aircraft types is modified - to military standards, the P-3 Orion is a militarised version of the Lockheed Electra of 1957. The original P-3A had structural improvements to be able to handle military loads. From the wiki:

While based on the same design philosophy as the Lockheed L-188 Electra, the aircraft was structurally different. The aircraft had 7 feet (2.1 m) less fuselage forward of the wings with an opening bomb bay, and a more pointed nose radome, distinctive tail "stinger" for detection of submarines by magnetic anomaly detector, wing hardpoints, and other internal, external, and airframe production technique enhancements.

The other aircraft is a C-130, a military aircraft as well. Military design specifications are for transporting heavy hardware such as tanks, and landing them on a rough runway under adverse circumstances.
Constant wind speeds are the natural environment of an aircraft. The danger to the structural integrity of an aircraft is in wind gusts: a sudden change in wind speed, which bends the wings, then accelerates the aircraft which brings relief to the bending moment. The wind gust results in a lift that is momentarily greater than the weight. This is expressed in the load factor n = L/W, which is a design standard for aircraft. Military transport aircraft have a limit load very similar to transport aircraft. From MIL-A-8861:

The P-3 was a submarine patrol aircraft and designed for load factors of +3.5/-1.0, the C-130 is a utility aircraft with n = +4.0/-2.0. Very similar to transport category aircraft - the storm chaser aircraft just have to avoid the wind gusts, and are otherwise OK in the storm.

The P-3 is a military variant, certainly, but the WP-3D isn't specially modified from the P-3 in any way to specifically enhance its performance envelope for use in turbulent weather.

Indeed, they're very capable and sturdy platforms. The P3-C's patrol around Australia, I read somewhere that the crew switches off a couple of engines while loitering. The Electra had a checkered history though.

One factor that makes the P3 Orion particularly suitable for hurricane duty... this aircraft derives a good deal of its lift from the airflow across the wings generated by the propellers. This is why it has fairly small wings for the size of the aircraft - the wash from the props add lift.
That makes it somewhat more resistant to wind shear, the sudden change of wind direction that can stall an aircraft in flight by removing the airflow over the wings. If a wind shear change slows airflow over the wings on the P3, the pilots can throttle up and get some lift and controllability back.


text:
What does it take to maintain a private pilot certificate?
What, if any, requirements are there to maintain a private pilot certificate? For example:

Do I have to renew my certificate after a period of time? E.g. annually, 5 years, 10 years
Do I have to receive a minimum number of continued training hours?
Do I have to fly a minimum number of hours?

For instance, say I had enough money/time to go out and start the process of getting a private pilot certificate tomorrow, but I didn't plan to make use of it in the foreseeable future, would it cost me more in time and money than if I waited until I had plans to use it?
This is assuming I live and fly in the USA.
Related Question:
How much, generally speaking, does it cost to learn how to fly?
Edit:
Note I understand the safety aspect of not practicing and "getting rusty", for lack of a better term on my part, and that practically a certain amount of retraining would be required (though it seems that it is not legally required). The retraining is not mandated and I would need to retrain until I felt proficient enough technically and practically. Retraining would include familiarizing myself with the controls, instruments, radio, physics, etc.
My main concern was that (a) at some point I my certificate would expire and I would have to go through the process again, making the initial effort practically useless, (b) that I may have to pay a significant amount of money to keep the certificate from expiring, or (c) that I would have to commit to a certain number of hours (instructed or solo) per year to keep the certificate.

Unfortunately I can't answer your question, because I'm not really familiar with the FARs yet, but please do not only consider the costs but also your own and other ones safety.

I found that if I flew less than about once a month that my landings would suffer, my review of checklists was slower, and my recall of emergency procedures was slower. However my navigation skills and knowledge of systems did not deteriorate. (Posting as a comment instead of an answer because it's a personal anecdote.)

Technical point, but it is not a license.  It is a certificate.

A license, once granted, is good forever (barring some enforcement action).
But, you need more than just a license to fly.  To use your license, while flying solo, you also need a Medical Certificate (if required for the aircraft or flight rules) and a Current BFR (Biennial Flight Review).
Medical Certificates are good for 5 years until you're 40, then good for 2 years.
Its a quick trip to an FAA approved doctor to examine you for any obvious medical conditions.
A BFR is a review with an instructor every two years to make sure you're still proficient and safe.  If all goes well, it takes about 90 minutes.
However, if you want to fly passengers, you also need 3 landings within the previous 90 days.
If you want to fly passengers at night, you need 3 landings to a full stop, at night, within the previous 90 days.
If your 90-day landings have lapsed, you can quickly fix it with 3 trips around the pattern, taking off and landing each time (touch-n-go during the day, or full-stop at night).  Then taxi back to the ramp, pick up your passengers, and go on your way.

I'd also add, these are the FAA legality minimums. In reality, you'd probably be dealing with an club or school, which will usually impose higher requirements for currency, per type, due mainly to insurance. These often are the 3 landings rules above every 30 days, per type.

BFR's are now just called Flight Review. A Flight Review requires a minimum one hour of flight training and one hour of ground review (61.56).

This needs to be updated for BasicMed.

The plain and simple answer to your updated question is that once you get your pilot license, it is good forever.  You can go 50 years without flying, and the license itself is still valid.
When it comes time to fly again, depending on the type of airplane that you want to fly, you will probably also need a medical.
You will need to have a Flight Review within the previous two years that you will need to be able to pass (and you have to be proficient in order to pass it).
Most importantly, in order to be safe, you will want to fly enough to be proficient and comfortable in the airplane so that you don't hurt yourself, others, or the airplane.  If you go a long time without flying, it can be almost like starting over.
In short, the additional training that you will have to do in order to become comfortable  will end up costing you more in the long run, but you don't have to do anything to maintain just the license portion that you are asking about.

@egid ...and you can get out of the boring "traditional" flight review if you're clever and participate in the WINGS program

There is no simple answer to this question because it depends how much you fly and how much time you need to re-learn after not flying for a while.
A US private pilot's license never expires, but you do lose currency. The recurring training requirement is in 14 CFR 61.56: you must pass a biennial flight review (BFR) every two years, which consists of one hour ground school and one hour in the air with an instructor. If you do not have a current BFR (or have passed an equivalent test) then you can't fly as PIC.
Apart from the BFR there is no requirement at all to fly regularly, nor is there a requirement to fly a minimum number of hours. That's for solo day VFR only; there are minimums if you intend to carry passengers, fly at night or operate under IFR (see 14 CFR 61.57).
So in an extreme case, you could get your license and not fly for 20 years. Then you would 'just' need to pass a BFR to become legal again, with no need to do the theory and practical tests over again (a BFR is not a test). But you would need a valid third-class medical as well.
The costs are impossible to estimate because it depends on how long your break from flying is and how quick you are to pick it up again. If you don't fly for a year you might need a couple of hours with an instructor to be safe again, but if you don't fly for 5 years it's probably going to be much more time. This is definitely one case where 'safe' is a lot more important than 'legal'.

It looks like you forgot a 3rd Class Medical in your answer.  Best-case, they last 5 years.  So after not flying for 20 years, you need a new Medical and a new BFR.

@abelenky Good point, I'll add that. Although if you wait twenty years the current proposed legislation to remove the 3rd class medical for VFR might actually have passed...

@abelenky And you're obviously younger than I am: my medical only lasts 2 years :-)

I just passed 40.... I made sure to renew my medical at 39.9, so it will now last me until almost 45. :)

All of the people here are correct.  Your license has no expiration date, but your medical does, depending on your age.
I personally went 15 years without flying.  This year I got my medical set and needed some time with an instructor to get the feel back for the plane.
I am back in the saddle after 7 hours with an instructor and the biennial flight review itself.

Thank you - this is a simple answer, but very informative. The amount of time it took you get comfortable again with an instructor is very helpful. How much flying did you do before the 15 years - were you amateur/commercial/military?

To answer your question simply with emphasis on continued cost.
Your certificate never expires and never needs renewed.
There is no minimum hours required for life.
The only cost involved to become current is to hire a flight instructor for a flight review(FR). This isn't required every 2 years, but you must have had a FR within 2 years of operating as a private pilot. There is no test and no examiner for a FR. The only requirement is one hour of flight and one hour of ground training with an instructor of your choosing.
The only other cost involved to operate as a private pilot would be a medical certificate from a designated medical examiner. A private pilot can fly as a sport pilot without a medical under certain limitations.
If you have the money/time now, and feel that it would more practical/better investment than waiting for costs to go up, than there is very little expense you would need to spend later.

The limits being flying a legal Part 103 ultralight plane then you would need no license or time in air or medical check to resume flying. I would advise you to take extreme caution as in any sport you lay off for a few years, you have a big surprise coming.. Use it or lose it ! The primery thought here is " Safety " yours & ours. So just generally follow the rules & the aviation community will breath a little easier .. RJ Muni

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text:
Why are there curved propellers?

Why are there curved propellers on some aircraft, what is there advantage?
If these are worthwhile advantages why aren't there curved blades on turbo fans?

@Pondlife my main question is could they be used in fans

why aren't there curved blades on turbo fans?

Some jet engines do have curved blades. https://en.wikipedia.org/wiki/General_Electric_GE90#/media/File:GE90_B777-200LR.JPG


text:
What is the FAA Approval Basis for an FMS upgrade on a Part 25 aircraft?
Does anyone know of a recent approval for an upgrade from a Universal UNS-xx FMS  to a Universal UNS-1Ew FMS on a FAA registered aircraft, other than a TC, TC update or an STC?
I am looking for the specific guidance which would allow a FAA Principal Inspector to do a Field Approval and/or sign the Flight Manual Supplement approval.

What aircraft type? What is the current certification basis (TC/STC)? Generally, the only way you can get an update approved by Field Approval is if there is already an existing cert basis (an STC) for the change or the change is minor. A change requiring a new AFMS is typically not considered a minor change, so you'll probably need the rights to use an existing STC for your aircraft type (assuming one exists). Have you tried asking Universal?


text:
Why do pilots disable the air-conditioning 'packs' during takeoff?
I've realized that some pilots turn the air conditioning units (also called 'packs') OFF during takeoff, like in this video:
LH456 Takeoff Video
Is there any operational reason for that?

it leaches power, they only turn it off for the inside engines

I also noted that both pilots didn't have their hands behind the throttle when pushing it to max power. Isn't that supposed to be done?

@shortstheory That's normal, because when they engage the Takeoff / Go Around mode, the throttle automatically moves to the correct position, so, they need to release, and right after the trust is correctly set, they hold the throttles again.

The "packs" (or A/C on the aircraft) are powered by engine bleed air. When bleed air is extracted from the engines, less thrust can be produced. This may be a problem if the airplane is taking off at a high elevation airport or the departure path requires a certain climb gradient to clear obstacles. This action does create some discomfort in the cabin so it's usually not preferred unless required for safety.
Some newer airplanes are manufactured with an "APU to pack" configuration - i.e. the pilots leave the APU running during takeoff, which provides bleed air to power the packs. The workload on the engines are now reduced.

I was wondering why they don't close the bleeds instead of the air conditioning packs? If they close the bleeds, there would be no loss, am I right?

Pilots will turn off the packs for takeoff if maximum engine performance is needed or if there is a need to prioritize a high demand bleed draw.  The particulars of the situations calling for a packs-off takeoff is airframe specific.
In the EMB-145, normal takeoff was with the APU bleed powering the packs.  The specific configuration calls for engine bleeds off, APU bleed on, cross-bleed valve open and packs on.  This lets the APU be the sole source of bleed air and the packs the sole consumer of bleed air.  This is a typical configuration, but assumes that the APU is operational and that anti-ice will not be needed.
If the APU is inoperative or anti-ice is called for, then the takeoff configuration becomes engine bleeds on, APU bleed off, cross-bleed off, packs off.  This isolates the APU from the bleed system (but keeps it available as a 5th generator) and makes the engines the bleed sources with no draw from the packs.  This configuration with the APU inop allows the engines to avoid a performance hit from the bleed air taps (drawing compressed air at the 9th and 14 compressor stages of this particular engine).  For anti-ice takeoffs, this configuration provides the ability to provide anti-ice as the APU bleed source is not powerful enough for this on the EMB-145.
The particular pneumatic configuration details will vary by airframe, and as mentioned in comments to the other answers, some airplanes have limitations on bleed valves being closed (e.g. to keep a pneumatic powered hydraulic pump operational) and some do not.
In summary, the reason for a packs off takeoff is likely for takeoff performance or for high demand needs such as anti-ice.

Best answer that I can think of, without the exact engineering or design info, is that since the A/C packs are run with engine bleed air, if you need the extra thrust from the engines, you'd turn off the packs for that extra power for take off...
When I did my dispatcher certificate, we used the 727... and from my notes, you'd get a 1500 lbs increase in runway limit performance, with the Packs off.

but why don't just turn off the bleeds, instead of the packs?

My guess would be how the system is designed, but I honestly don't know.

No, you can never turn off the bleeds of an engine. The flight control surfaces are powered by hydraulics, which are powered by bleed air. Disabling bleed air would result in very degraded maneuverability (minimal control is provided by backup systems).

@YgorMontenegro On the 747-100 and -200 you can turn off the bleeds individually by engine. However, you would be removing some of the hydraulic power redundancy. Each engine has two hydraulic pumps, one powered by the engine itself and one powered by the pneumatic duct, which is pressurized by the engine bleeds. That latter pump is not brought into play as I remember unless hydraulic pressure from the engine pump drops below a set value.

@kayson Not true on all aircraft. On the EMB-145 we could open and close the bleed valves and our hydraulic backup pumps were electric, not pneumatic.

@casey so the only function of bleed air is to pressurize the cabin in the EMB-145 (thinking if you can control the bleed valves)? what about de-ice?

@kayson starts, packs/pressurization and a/ice. If we took off into icing we used a engine bleeds on, crossbleed off, apu bleed off, packs off config.


text:
How does terrain alert system work?
How does the voice in landing phase figure out the altitude from the runway? Is it reading the altitude from altimeter or totally separate system?
How accurate is the reading?
Where is the reference point for altitude measurement? is it from landing gears?

The system receives altitude from multiple sources, but the source for the callouts that you reference is the radar altimeter. Yes, it is calibrated to read 0 when the aircraft touches down. (I don't know how accurate they are.)

If you talk about ILS I believe that the voice announce horizontal distance from runway.

For altitude callouts during the landing phase (2500, 1000, 500, 400, ... 100, 50, 20, 10), the computer simply reads the data from the radar altimeter.
The radar altimeter uses radio waves to measure the vertical distance between the airplane and any ground / obstacle below. The radio altimeter is mounted on the underside of the fuselage. They are usually accurate up to feet. This is different than the standard altimeter in the cockpit, which measure air pressure.
The radio altimeter has no idea where the runway is; if there is uneven terrain leading to the airport, or if the pilots' descend profile does not match the glideslope, the readings (and thus the callouts) will not reflect the height above runway elevation.
Note that the airlines can customize the altitude callouts. Different manufacturers also uses slightly different callout patterns.

The second type of audio alerts are messages from a Terrain Awareness and Warning System (TAWS). These systems uses radio altimeter, and may combine information from other sources, such as GPS and a global terrain database, to issue alert messages to the pilots if terrain impact is imminent.
Since the radio altimeter is only capable of detecting ground directly below, it is unable to provide timely warnings in certain situations, e.g. approaching a steep hill. By integrating with GPS data and a terrain database, the computers can predict the flight path of the aircraft and issue warnings if a risk of collision exists.


text:
Why do passenger embark on the left side of an aircraft?
I speak of passenger aircraft that can be found on large airports.
The left side is used by passengers (and the crew) to embark/disembark (see the position of jet bridges) and the other side to embark/disembark the rest (e.g. food).
Why do we always use the left side for passengers? What prevents an airport from deciding to use the other side?
If this is an historical decision (e.g. one day in the past an aircraft builder decided passenger should use the left side, then majors airports were built accordingly and thus new aircraft were built accordingly and thus new airports were built accordingly), is it possible and reasonnable to build a jet bridge made to operate on the right side of an aircraft and then use the right side for passengers and the left side for the rest?

Duplicate: Why do airplanes at every airport disembark at the left (port) side?

Well I do it because that's where the jetbridge is. Whenever I try to embark on the right side I can't reach the door.

Why do passenger embark on the left side of an aircraft?

Because the cavewoman who invented the log-canoe was right-handed?
Note: I arrived at this answer by looking at a kind of etymology of the terms starboard & port but that's incidental. The answer to this question happened to be contained in an answer to a rather different question. Some groups of aviators may have dropped the archaic terms while still being subject to a continuation of an archaic habit/convention.
People get on the left side of a jet airplane because Vikings (etc) steered their wooden boats with a steering board (hence starboard?) held in their dominant right-hand.
Since it was inconvenient to have your steering apparatus wedged against a quayside (or earthen bank) they docked at the port along their left side and therefore loaded and unloaded on the left.
Farhan's comment leads you to this answer in English.se which I reproduce here for convenience
Excerpt from trivia on the Navy website:

Port and starboard Port and starboard are shipboard terms for left and
  right, respectively. Confusing those two could cause a ship wreck. In
  Old England, the starboard was the steering paddle or rudder, and
  ships were always steered from the right side on the back of the
  vessel. Larboard referred to the left side, the side on which the ship
  was loaded. So how did larboard become port? Shouted over the noise of
  the wind and the waves, larboard and starboard sounded too much alike.
  The word port means the opening in the "left" side of the ship from
  which cargo was unloaded. Sailors eventually started using the term to
  refer to that side of the ship. Use of the term "port" was officially
  adopted by the U.S. Navy by General Order, 18 February 1846.

So it's a historical convention, many arbitrary choices are of this sort. Pioneers of a new form of transport adopt the terms (port, startboard) and habits of more ancient forms of transport.

Ironically, since aircraft became able to carry substantial non-human cargo, the cargo entry has moved to the other side to reduce the chances of crushing the human cargo with the non-human cargo. Presumably also because having lots of large holes all in one side may be structurally disadvantageous.

As an ROTC cadet and private pilot, I must say, I have never legitimately used the terms port and starboard.

@KeeganMcCarthy: The terms “starboard” and “port” were and may still be used in RAF and US Navy, but not so much elsewhere and certainly not in civil aviation.

@JanHudec Isn't there something about the Navy NOT using the terms for their aircraft because once aircraft is carried on ships, it would be confusing? Or is that just a QI fact?

@biziclop: I don't know really. I know RAF used it, but don't know whether they still do and I don't know whether Navy applies or applied them to aircraft or only to ships.

Another entertaining story on ancient technology determining the modern: "..the major design feature of [the space shuttle] was determined over two thousand years ago by the width of a Horse's Ass" link

@aitchnyu That link specifically says that the quoted claim isn't true... funny as it may be.

Interestingly enough, even though cargo is loaded on the right-hand side for passengers planes, most freight airliners (such as B767-F, B777-F) are loaded with cargo from the "port" side.

Early fighters with a cockpit (Supermarine Spitfire, Messerschmitt 109) had cockpits which were hinged on one side. For a right handed pilot (the majority), it was easier to lift the canopy over your head and to the right, hence the hinge was on the right. It's also easier to climb up an aircraft if your stronger arm is toward the fuselage and handholds.
This made the left the obvious choice for disembarking, and since airfields slowly became set up for exiting to the left and fuel to the right, newer designs maintained the door on the left. If your stands are set up for one direction, you may as well order your new plane to fit the same workflow, so even when hoods were modified to slide backward, there was no need to change the door side (most designs were incremental, so the door was already there... Why move what already works?)
This continued as air bridges and similar were introduced, and it would now be pointless to make an aircraft with doors on the other side, as you'd just be increasing your loading/turnaround times.

The Spitfire and ME109, however, were inter-war designs, and passenger aircraft were available before that. Also, every commercial airliner I've been on does have a door on the right, straight across from the door I've entered on the left. It's used by catering to get food on/off the plane as well as the cleaning crew entrance/exit. Therefore, it's not really "pointless to make an aircraft with doors on the other side".

I expect it comes from the fact that aircraft development started in countries that drive on the right side of the road. The pilot sat on the left because, well, that's where drivers sit. The early airliners would park with the "terminal" / shed / tent / hole in the fence on the left side so the pilot could steer without hitting anything (better view out the window), the pilot could climb out his door and open the rear door to unload the mail without walking around to the other side of the plane so that's where the cargo handlers / trucks / horses would wait.
Remove the front door on larger aircraft, replace it with a rear door closer to the ground. You still put it on the left for compatibility with all the other stuff flying. Fast forward to the modern day and we find that we have to pick one side or the other, everything has a left door, some have doors on both sides, so....
I've considered that monsters like the A380 should use jetways on both sides but making a jetway reversible is mechanically very complex. Economics kick in here as the largest planes are not used for short routes where a 30-minute turnaround matters. The cost of dual-side jetways doesn't have a reasonable payoff.

Just FYI, the A380 is typically loaded with three jetways -two for the lower deck, one for the upper deck.

@BurhanKhalid but all from the left.

I have a hunch this is because the cargo doors on non-cargo aircraft are on the opposite side.

My $0.02 worth: This may be right but it seems to be pure speculation. Some supportive historical references would be useful.

I don't buy this. You're saying that it's because planes evolved in countries that drive on the right but parking with the left side of the vehicle at the side of the road" is a property of countries that drive on the left!

@DavidRicherby Cars park on the right so the driver can see approaching traffic when he pulls out. Planes parked with the building on the left so they don't hit it with the wings. In both cases it's operator visibility of the most hazardous obstacle. The curb can take a few tire hits. So can the tire.

The earliest air forces in Europe derived from cavalry regiments, flying was expensive and the preserve of the rich - as was the cavalry.
Because a right handed horseman carries his sword on the left, he will mount a horse from it's left side - so he throws his right leg over the mount.
This is why the pilot in command sits in the left seat and enters from the left side of the aeroplane. When passengers started entering through a door the same tradition continued.

Citation needed.  In early planes, the pilot sat in the centre and that probably lasted for long enough for them to forget they used to be in the cavalry.

But they stepped aboard from the left. Some airforces still haven't forgotten they are cavalry, having warrant officers rather than sergeants for example


text:
Why are 4 blade propellers often in X or + position when the aircraft is parked?
Is there a meaning to the position of 4 blade propellers on parked warbirds?
A lot of old pictures have them at X or + positions.

I have operated twin engine aircraft with four bladed propellers. While not warbirds, the following should illustrate some of the reasoning that goes into positioning aircraft propellers. Each type and situation could vary.
In our operation we have two positions in which we place the blades when the aircraft is parked. The positions are the x and + position you mention, and we will choose the most appropriate one dependent on the situation.
The + Position
When we park such an aircraft on a ramp we may want to secure the propellers so that they cannot turn in the wind. One common method of securing the propellers involves placing the propeller in the + position and securing the bottom blade with a cuff that is tied off to the sides with a bungee.
These propellers are bolted to PT6 series engines in which the propeller section (power turbine and gear box) is free-turning. Securing the prop helps ensure that the gear boxes do not turn without proper lubrication, and helps reduce wear.
The x Position
When the aircraft is parked in a hangar or on a ramp with the propellers unsecured, which is more common for our operation, we want to place the propeller blades in the x position.
This is to put the blades in a position which poses the least risk to people moving about the aircraft. The height at which the blades would sit in the + position would typically put the blade tip right at head level, a dangerous position for a relatively sharp and low visibility aircraft part. Conversely, placing the blades in the x position places the upper blade tips out of the way and the lower blade tips down near knee level.

I had seen somewhere that it's because the cowling has a hole to let water drain, and you want to position that straight down -- which usually corresponds to the + or x positions, depending on the cowling. Is that not true?

@yshavit No. The cowling is fixed to the airframe (except in old, rare probably-two-bladed designs).

I probably used the wrong word. I meant the cone that sits right on the prop's axis.

@yshavit Well, there could be some that are designed that way, though it could create balance issues. In my years of flying and working on airplanes I cannot recall a spinner with a drain hole. Now, that being said, many old wooden propellers have drain holes at the tip, and I could see perhaps positioning a two bladed prop straight up and down partially for the purpose of letting the one blade drain.

Hm, interesting. I believe the plane I specifically heard that about was an LC-130 being used in antarctica. So I wonder if it had a modified spinner because the risk of water pooling in it and then freezing (which would itself cause balance issues) is greater. Or maybe I'm misremembering!

@yshavit You may well be right. I don't have any experience with that type.

I crewed on C-130s for 4 years, we did it strictly for appearance.  It's a military thing...  ;)

If freezing rain is expected,
put it in + position so one of the spinner's blade exit slots
points straight down, to completely drain the spinner to avoid ice buildup therein.
If you have a square-tip propeller, don't put the topmost
blade exactly vertical, so birds perched on it
don't make their acidic poop run down the length of the blade.
-- Paraphrased from S. Lind, 2019, Small Aircraft Operations Manual.


text:
What is the significance of the "date designation expires" on a temporary airman certificate?
I recently received my temporary airman certificate and it says that it expires in 120 days. However, there is a section that says "date designation expires". The date under "date designation expires" is not 120 days. So my question is, does it expire 120 days after the date of issuance or on the date under "date designation expires"? Thanks for any help!

This answer assumes you are in the United States.
I can't find one of my temporary certificates sitting around at this moment, but I'm 97.2% sure that Date Designation Expires is the date that the examiner or inspector who gave you the certificate's designation expires.
I can 100% guarantee you that your temporary certificate expires 120 days after the date of issuance, unless it is otherwise renewed, superseded, or revoked as stated in the limitations on the certificate itself.


text:
What is the procedure to follow for "traffic not in sight"?
What would be the proper procedure to follow, when in the pattern or approaching an airport, if ATC gives you the position of an aircraft and you do not have them in sight? Do you just report "not in sight" and wait to turn base?

Please don't use "Not in sight". Parts of radio transmissions, such as single words, can easily be cut off or missed, making it sound like you said "In sight". I commonly hear pilots use the phrase "Looking out" or something similar, which conveys a clear message: "I understood what you said, but I don't see the traffic yet".
ATC provides traffic information when there is a risk of conflict, or in the traffic circuit if we want you to follow another aircraft. No matter the reason, we will continue to update you on the traffic either until it is no longer a factor, or until you report having the traffic in sight. So, when you receive traffic information, keep looking until you see the traffic. We will keep you updated as long as it is still relevant. And please do report when you spot the traffic, that is a super helpful piece of information to us.
In the traffic circuit specifically, if you can't see the traffic being called out, you can always ask the tower to confirm you are cleared for base and final turn. When I have someone on downwind who can't spot the aircraft to follow, I will typically just tell them to continue downwind and then I will call their base turn when appropriate. But, as always, if in doubt, ask.

The correct phrasology when receiving a traffic service in the UK is Traffic not sighted. This doesnt reconcile well with your comment (which is of course true) about the odd word sometimes getting dropped.

@Jamiec That phraseology is UK specific. It does not comply with ICAO SARPS: https://www.skybrary.aero/index.php/Traffic_Information

In the USA, we attended a safety seminar, where the preferred response was "Negative contact, Nxxxxx", or "Traffic in sight, Nxxxxx".
My panel now shows ADS-B In traffic, so planes with ADS-B Out are a lot easier to spot as we now have a much better idea of where to look.


text:
How to calculate the variation of Z force with elevator deflection (Zδe) in Nelson for pitching dynamics?
I have been trying to model the pitching dynamics of a General Aircraft (Nelson, "Flight Stability and Control").  I need to use the Zδe which uses CZδe parameter in the control matrix B, but Nelson provides no method to find it.  The state space equations are:
]2

There are a couple ways you can estimate control derivatives ($C_{L_{\delta e}}$ is a control derivative). You can then transform the coefficient from stability-axis to body-axis for use in the state space equations of motion.
Elevator is basically a full-span or part-span plain flap. Knowing that, you have several tools at your disposal:

USAF DATCOM: It contains semi-analytical equations to estimate 2D flap lift increment, and corrections to 3D with aspect ratio and taper ratio. Not the easiest to read resource, but it is free and forms the backbone of many conceptual design software and books.
ESDU: Items 70011 and 74011 contains semi-empirical relationships for estimating full-span 3D flap control derivative. ESDU contains step-by-step examples and is the authoritative source for all things aerospace in conceptual design and validation. Unfortunately, it isn't free.

Before going further, I should mention that tail experiences downwash from the wing. So converting from the tail stability-axis to aircraft body-axis requires converting the total tail incidence angle ($\alpha_t = \alpha+i_t-\epsilon$). Of course, you'll need to find downwash itself, which would be the topic of another question.

Vortex-Lattice Method: AVL is a reliable and free VLM, and is the go-to tool for conceptual low subsonic aerodynamics. Good thing about it is you don't necessarily need to estimate downwash; it directly outputs the control derivatives in aircraft stability-axis.

For conventional layout, all three tools should give you similar answers. Otherwise, you may not know the true value until you have access to wind-tunnel.

The variation of Z-force with elevator deflection is simply the component in Z-direction of the lift force developed by the elevator. It is a function of:

Local Angle of Attack.
Elevator deflection angle.
Trim tab or stabiliser deflection.
Elevator surface area.
Local dynamic pressure at the elevator.


text:
What prevents a government from selling/leasing publicly owned airports?
When an airport is open to the public, it is generally owned and operated by a government entity. If the local government leases or sells the airport, this would generate significant amounts of revenue.
What are the legal impediments which discourage a local government (who owns a public airport) from leasing or selling their airport to a private entity?

Not necessarily true -- privately owned public use airports are a thing in many parts of the US, and most of them have been privately owned since their inception.

@UnrecognizedFallingObject I see what you're saying. I was just giving a background to my question, because that's the way it is in my law homework.

Which country are you talking about ?

USA. It's in the tags

For a good example you can read about the ongoing battle over Santa Monica; the city wants to close it but the federal government says they can't. This article has some details of the legal arguments and you can Google plenty more.

This might get a better answer on the Law Stack Exchange site, since you're asking specifically about legal impediments to the sale/leasing of an airport.

One possible impediment might be the grant assurances that come with accepting Federal aid to improve the airport.
In short, if the local airport accepts money from the Federal government to improve the airport, that money is often granted "with strings attached" - requirements that the airport remain open, etc.

I did consider the grant agreement, however, the specific circumstances (from my law textbook) do not mention any AIP federal funding. So I'm thinking its referring to a state or local government independently owning a public airport.

You can buy a stake from an airport, Google  "China buys Toulouse airport"

This is probably what the book is looking for as an answer: It is the aspect that makes sale of the airport least attractive to a developer. It's worth noting however that grant assurance obligations would really only be an "impediment" if the buyer intends to cease operating the land as an airport and redevelop it for other purposes (housing, shopping, etc.) - If the buyer intends to operate the airport as a going concern a requirement that it continue to operate as an airport wouldn't really impact their plans.

Some places have land laws that protect public spaces as well as Privately Owned Public Spaces. For example there are Privately Owned Public Use Airports in the US like Trenton-Robbinsville N87. In some cases the laws may not prevent the rental to a private organization but may require that the airport remain in public use. IN this case the government may not want to rent/sell it since it needs to remain a public entity anyway.

All US aircraft and airfields are subject to militarization on a moments notice. It's an old Cold War law that's still on books. In NATO planning, virtually the entire private airliner fleet was going to be militarized to form an airbridge if the Russkies invaded.(It was assumed that nuclear attack subs would be way worse on ocean shipping than German diesel-electric were in WWII.)
Some airports are designated emergency military landing zones. Used to be this little grass stripped airport outside of Bangs, Tx that had to sweat to land an ultra-light but still had an official notice on the wall that it was expected to handle military aircraft during a designated emergency. I finally figured out they meant helicopters from the nearby National Guard training ground.

authors: Владимир Русских